Steered beam ultrasonic sensor for object location and classification

ABSTRACT

A vehicle occupant sensor which utilizes an acoustic system for determining object range, extent, and direction. The system is composed of an ultrasonic transmitter formed from an array of air chamber resonator elements driven in relative phase to each other to produce a steered acoustic beam. Electrical excitation circuitry converts waveform data stored in memory elements to electrical signals to drive the transmitter. One or more ultrasonic receivers receives the acoustic waves reflected from objects, and a processor determines the range, extent and direction of the objects based on the received acoustic waves.

FIELD OF THE INVENTION

The invention relates in general to an apparatus and method fordetecting the presence of an object within a compartment of a vehicle.More specifically, the invention relates to an apparatus and method thatuses an electronically steered ultrasonic beam to measure the range,angular extent and angular direction of an object located within acompartment.

BACKGROUND OF THE INVENTION

Air bag systems have become a standard vehicle safety feature to preventinjury to vehicle occupants. Unfortunately, in certain circumstances,early first generation air bags sometimes caused injury to the vehicleoccupants due to the indiscriminate nature in which the air bags wereinflated. The air bags would inflate at maximum force regardless as towhether the occupant was a child or an adult, whether the occupant wasproperly seated to face the air bag, or whether the occupant was tooclose to the point of air bag deployment. The application of maximumdeployment force of the air bags to children or adults of small staturehas resulted in injury even in relatively low speed collisions.

In view of the problems associated with the first generation air bagsystems, a variety of “smart” air bag systems have been developed in anattempt to prevent unwanted injuries from occurring due to air bagdeployment. These second generation air bag systems include sensors fordetecting the presence of an occupant within a vehicle. U.S. Pat. No.5,906,393 issued to Mazur et al., for example, discloses a system inwhich a weight sensor is used to determine the presence of an occupantin a vehicle seat. Other systems have been developed to specificallydetect the presence of a child seat. U.S. Pat. No. 5,901,978 issued toBreed et al., for example, discloses a system for detecting the presenceof a child seat that utilizes ultrasonic transducers.

While the above-described systems are improvements over the firstgeneration systems, they are generally limited in the amount ofinformation they can provide to control air bag deployment. It would bepreferable to provide a system that could detect not only the presenceof an occupant, but also distance of the occupant from the air bag andthe occupant's angular direction and angular extent. For example, aschildren are generally narrower in width than adults, it would bebeneficial to provide some measure of the angular extent of the occupantto provide a simple method of determining if the occupant is a child oran adult of small stature.

Ultrasonic or acoustic range finding in itself has been applied in manyapplications including, for example, lens focusing systems for incameras in which an ultrasonic range finder computes the distance to anobject and adjusts the lens focus accordingly. In such acoustic rangefinding applications, an appropriate transducer generates an acousticsignal as a short duration pulse. The pulse is reflected off of nearbyobjects and is received by the same, or another, transducer. As thespeed of sound in air is a known quantity, the distance of the objectfrom the transducers can be calculated from the transit time of theacoustic pulse.

Ultrasonic range finders typically use ultrasonic frequencies which areinaudible to the human ear. These high frequencies have inherentlyshorter wavelengths, which lead to greater positional accuracy thanaudible frequencies. Some systems known in the art use severalsimultaneous signals with differing frequencies. These simultaneoussignals are generated to provide at least one readable signal in thepresence of acoustic interference.

Ultrasonic sensors are typically made from a single transmitter/receivertransducer. A brief ultrasonic pulse is transmitted, and this isreflected from a nearby object. The transducer, now used as a receiver,detects the reflected pulse. This type of sensor will give objectdistance information, but provides no angular position or extentinformation.

Phased array radar systems utilize a stationary array of transducers togenerate object distance, angular extent, and angular positioninformation. An array of many transducers driven at different amplitudesand phases can produce a lobe pattern of one narrow beam which issteerable over a wide angle. This technique is called aperture synthesisand it is used in phased array radar systems.

The beam is formed by the interference of the radar waves, a consequenceof the principal of linear superposition. In linear superposition, theradiation of one source combines with that of another source to eitherincrease or decrease the radiation amplitude at a point, causingconstructive interference or destructive interference respectively. Awell known example of this principal is the Young Experiment of 1802 inwhich light is passed through a pinhole to create a point light source,then it is through two other pin holes, finally the light is projectedon a screen. A regular pattern of light and dark bands appears on thescreen, which is caused by the interference of the two point sources. Amore detailed analysis of the Young Experiment appears in D. Hallidayand R. Resnick, “Physics for Students of Science and Engineering,” PartII, Second Edition, John Wiley & Sons, Inc., New York, 1962, pp.976-982. Further information may be found in Grant R. Fowles,“Introduction to Modern Optics,” Holt, Rinehart and Winston, Inc., NewYork, 1968, pp. 62-66. The linear superposition effect is applicable tolight waves, radar waves, and acoustic waves.

A similar steered beam system which uses a stationary array oftransducers would be desirable for vehicle occupant detection. However,due to the inherent characteristics of radar wavelength and frequency,radar is not accurate enough for close range use in measuring therelatively small variations in distance between a passenger and anautomotive air bag. Therefore, a device which uses aperture synthesistechnology and facilitates accurate short range distance measurement isneeded.

If an array of acoustic transducers were utilized, an interferencepattern could be formed if the transducer spacing is about the same asthe wavelength of the acoustic signal. If the speed of sound in air isabout 344 m/sec, an ultrasonic transducer operating at a frequency of68.8 kHz will have a wavelength of 5 mm, while higher frequencies willhave proportionately smaller wavelengths. If two transducers are usedand spaced apart by a wavelength, the transmitted beam pattern will besimilar to the beam pattern shown in FIG. 1 in which a central main lobeis produced with corresponding side lobes. If a phase shift from 0-180degrees is introduced between the transducers, the main lobe is steeredto one side and the intensity of the side lobes is changed until asymmetric lobe pattern is achieved at a 180 degree phase shift as shownin FIG. 2. Thus, a simple two transducer system could offer limitedscanning capability and the ability to distinguish between objects atthe front and sides of the transducers. A larger array of transducers,for example a 4×4 array, driven at different amplitudes and phases couldproduce a lobe pattern including one narrow beam steerable over a wideangle as illustrated in FIG. 3.

While efficient ultrasonic transducers are available commercially, thesetransducers are typically formed as a resonant diaphragm with diameterof about one wavelength since this structure produces an efficientconversion of electrical energy to sound energy. The speed of sound in atypical diaphragm material is much faster than the speed of sound inair. As a consequence, the wavelength in the diaphragm is larger thanthat in air at the same frequency, and the diameter of these diaphragmtransducers is larger than one wavelength in air. Thus, the closestpossible spacing of this type of transducer exceeds one wavelength inair.

The inherent diameter of the diaphragm transducers presents a problemwhen constructing an array of acoustic sources to generate a steeredradiant beam. In the simple case of two sources separated by a spacing dand driven in phase by the same excitation voltage, the angularseparation in radians between maxima or minima of the interferencepattern is about λ/d. For a transducer separation of twice thewavelength, the resultant interference pattern has a beam perpendicularto the plane of the transducers defined by minima at 14.3 degrees ateither side of the center. A pair of secondary side lobe beams areformed at 28.6 degrees on either side of the central beam. If it isdesired to probe only the region directly in front of and perpendicularto the plane of the transducer, there will be undesirable interferingsignals from these secondary side lobe beams and additional side lobebeams at larger angles. FIG. 4. illustrates angle spacing to the firstminimum as a function of separation of two acoustic sources.

As described above, a desirable method of producing a steered acousticbeams is to use an array of many transducers driven at different signalphases. The beams from the individual transducers will supplement eachother to give a large signal amplitude in a given direction. Destructiveinterference between the beams will lead to a small signal amplitudeaway from the chosen direction. In the two transducer case, however, thepresence of side lobes will lead to ambiguous signals. In range findingapplications, accurate positional information will be lost sinceacoustic reflections would be generated by the side lobes as well as themain beam. A solution to the side lobe beam problem would be to placethe array transducers very close to each other, preferably less than onewavelength. Unfortunately this is impossible for the aforementionedresonant diaphragm type of transducer, due to the inherent diameter ofthe transducer which is larger than one wavelength.

In view of the above, it is an object of the invention to provide anapparatus and method that utilizes a steered acoustic beam for objectlocation and classification, and further to incorporate such apparatusand method into a detection system for detecting the presence of anobject within a compartment of a vehicle, as well as the objects'sdistance, angular direction and angular extent.

SUMMARY OF THE INVENTION

The present invention provides a vehicle occupant sensor which utilizesan electronically steered acoustic beam to scan its surroundings in asweeping or spot pointing pattern to determine the angular position,angular extent, and range of objects. The steered beam is provided by astationary array of acoustic sources driven in relative phase to eachother. The apparatus employs aperture synthesis techniques which arecommon in phased array radar systems, but have not been applied toacoustic systems. Undesirable side lobe beam interference is reduced bythe utilization of air chamber resonators, which permit the system toperform as if the transducers were spaced less than one wavelengthapart.

More specifically, an acoustic wave transmitter is provided thatincludes a resonator body, a plurality of air chamber resonatorsarranged in an array in the resonator body to form an array of exitholes, and a plurality of transducers corresponding to the plurality ofair chamber resonators. The transducers are acoustically coupled to theair chamber resonators and drive the air chamber resonators to generatean acoustic signal, and the spacing provided between adjacent exit holesis not greater than one wavelength of the acoustic signal.

The air chamber resonators preferably extend through the resonator bodyto form a second array of exit holes. A tuning mechanism, in the form ofmoveable plugs, is provided in the exit holes of the second array. Thetuning mechanism permits the tuning of the resonance and phase of theair chamber.

In order to operate the transducers, a control circuit is provided thatconverts waveform data stored in a memory to electrical drive signalsthat are supplied to the transducers, wherein the transducers drive thechamber resonators in relative phase to each other to produce a steeredacoustic beam as the acoustic signal. The control circuit preferablyincludes an addressable memory containing digital waveform data fordriving each air chamber resonator, a register for selecting regions ofthe addressable memory in which the waveform data reside, and a counterfor cycling through the selected regions to present the waveform data tothe transducers at a desired rate.

The transducers are preferably vibrating diaphragm type transducers, andare acoustically coupled to the air chamber resonators via transducerexcitation holes formed in the resonator body. The transducer excitationholes can be formed on a common side of the resonator body or onopposite sides of the resonator body.

An input transducer or receiver is used to receive the acoustic signalgenerated by the air chamber resonators after it has reflected off anobject. A processor coupled to the receiver determines at least one ofrange, angular extent and angular direction of objects that reflect theacoustic signal, based on the signal received by the receiver.

The invention is preferably incorporated as a detection apparatus fordetecting an object within a compartment of a vehicle, such that theacoustic wave transmitter is located to scan a steered acoustic signalwithin the compartment, and the receiver receives a reflected acousticsignal generated by the reflection of the steered acoustic signal offobjects located within the compartment. A controller determines at leastone of range, angular extent and angular direction of the object, andpreferably controls the operation of an air bag in response thereto.

Other advantages and features of the invention will become apparent fromthe following detailed description of the preferred embodiments of theinvention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to certain preferredembodiments thereof with reference to the accompanying drawings,wherein:

FIG. 1 is a beam pattern for two transducers a wavelength apart whendriven in phase;

FIG. 2 is a beam pattern for two transducers a wavelength apart whendriven out of phase;

FIG. 3 is a beam pattern for an 4×4 array of transducers;

FIG. 4 is a graph of the calculated angle of appearance of the firstamplitude minimum as a function of source transducer spacing

FIG. 5 is a schematic block diagram of an apparatus in accordance withthe invention;

FIG. 6 is a perspective view of a resonator body with transducerexcitation holes provided on a common side;

FIG. 7 is a perspective view of a resonator body with transducerexcitation holes provided on opposite sides;

FIG. 8 is a perspective view of transducers mounted on the same side ofthe resonator solid body;

FIG. 9 shows a plan view an embodiment of the linear acoustic resonatorarray with exit holes spaced for 41 kHz operation;

FIG. 10 is a graph of square wave electrical signals in a memory elementaddress space for driving phased array transducers at selected relativephases;

FIG. 11 is an electrical schematic of an embodiment of the electricalsignal generation circuit for driving transducers:

FIG. 12 is a graph illustrating acoustic beam steering ability in a 41kHz linear resonator acoustic source array of eight elements;

FIG. 13 is a graph of reflected acoustic energy from room objects as afunction of range and beam angle for a 41 kHz linear resonator acousticsource array of eight elements; and

FIG. 14 is a schematic diagram illustrating an apparatus for detectingan object within a compartment of a vehicle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention uses a stationary array of acoustic sources togenerate a steered acoustic beam. The steered acoustic beam performs anangular or spot pointing sweep of its surroundings to determine theangular position, angular extent, and distance of vehicle passengers.The steered beam is provided by an array of acoustic transducers drivenin relative phase to each other. The beam is formed by the interferenceof the sound waves, a consequence of the principal of linearsuperposition. With proper choice of phases, the beams from theindividual transducers supplement each other to give a large signalamplitude in a given direction. A multiplicity of acoustic sources hasthe simultaneous advantage of reducing the beam width and expanding theangular sweep range of the beam. Aperture synthesis is achieved byindependently controlling the phase and amplitude of each acousticsource.

Due to the inherent diameter of diaphragm transducers discussed above,they cannot be spaced more closely than about one wavelength apart.Unfortunately transducers whose spacing is greater than one wavelengthwill generate undesirable side lobe beams. The present inventionovercomes this spacing problem through use of an array of resonant airchambers driven by resonant diaphragm transducers. These chambers permitthe transducers to perform as if they are spaced more closely than isphysically possible. Therefore, destructive interference between thebeams to minimize the magnitude of the undesirable side lobe beams.

Referring now to FIG. 5, a basic schematic block diagram of an acousticphased array vehicle occupant sensor system 10 in accordance with theinvention includes an array of acoustic output transducers 12 coupled toa phase shifter 14 that is driven by a ultrasonic oscillator 16 undercontrol of a microprocessor 18. An input transducer 20 is provided toreceive acoustic signals reflected from objects to be detected. Theinput transducer 20 is coupled to a digital signal processor 22 whichextracts the reflected signals from background noise and digitizes thesignals for further processing by the microprocessor 18.

The structure of the acoustic output transducers 12 will now bedescribed in greater detail. As shown in FIG. 6, a plurality of airchamber resonators 24 are formed in a solid body 26 such that the airchamber resonators 24 extend from a first end 28 of the solid body 26 toa second end 30. The air chamber resonators 24 are tapped by formingtransducer excitation holes 32 perpendicular to the air chamberresonators 24 from either side of the solid body 26 to allow transducerexcitation of the air chamber resonators 24. Although circularcross-sections are shown, the air chamber resonators 24 can be formed ofany desired shape. The air chamber resonators 24 are spaced so that afirst array of exit holes 34 are provided on the first end 28 of thesolid body 26 and a second array of exit holes 36 are provided on thesecond end 30 of the solid body 26. The exit holes 24, 26 are spaced ata particular fraction of the wavelength of acoustic waves to begenerated.

FIG. 7 illustrates a second embodiment in which the transducerexcitation holes 32 are present on opposite surfaces of the solid body26, which allows for a closer spacing of the air chamber resonators 24in some circumstances. As in FIG. 6, the air chamber resonators 24 forma first array of exit holes 34 at the first end 28 of the solid body 26,and a second array of exit holes 36 at the second end 30.

FIG. 8 illustrates the functional elements of the operational linearacoustic resonator array. Transducer elements 38 are mounted above thetransducer excitation holes 32 to cause acoustical resonance in the airchamber resonators 24. The second array of exit holes 36 are closed offat the second side 30 by moveable plugs 40 which allow for fine tuningof the air chamber resonance and a phase adjustment. In a preferredembodiment, a portion of the moveable plugs 40 and air chamberresonators 24 are threaded to allow for an easy phase adjustment.

FIG. 9 illustrates the spacing of the exit holes 34 in an embodiment ofa linear acoustic resonator array for a 41 kHz operating frequency. Forsuch an array to form a steered beam, it is necessary for each elementof the array to be driven at a particular signal phase which is fixed bythe desired beam direction. In the present invention, the drivecircuitry has been simplified by noting that resonator type transducersdo not require a sine wave excitation.

Instead of a sine wave excitation, these transducers 38 are excited attheir resonant frequency by a square wave voltage signal. When a squarewave voltage signal is decomposed into a summation of odd harmonic sinewave signals by the Fourier transform, it is seen that there isconsiderable signal energy at the first harmonic fundamental frequencyand decreasing signal energies at the subsequent harmonics. A squarewave electrical signal at the transducer resonant frequency will excitethe transducer with nearly the same efficiency as a sine wave signal.

In the present invention, these square wave signals are preferablystored in a digital memory element as graphed in FIG. 10. The squarewaveform data are stored in the memory element as a sequence of digitalbits. When the memory address is repeatedly cycled from its startingaddress (0) and its maximum address (N), the bit outputs become timevarying square wave signals of period (N/f), where f is the clockingfrequency of the address. As shown in FIG. 10, these bit images of thewaveforms can be set to maintain a relative phase shift with respect toeach other. Although only eight bit waveforms are shown, the number ofpossible waveforms is not limited, and such a scheme can drive anydesired number of transducers.

The circuitry for generating square wave signals of different relativephases for each transducer is also simplified when only a fixed numberof beam directions is required. In this case, the waveforms specifying aparticular beam direction can be stored as “pages” in the memoryelement. A practical embodiment of a circuit for accomplishing this isshown in FIG. 11, in which the digital waveform data for driving eighttransducers are stored in eight pages of 128 memory locations each. Thememory space of each page, 128 address locations, is sufficient tospecify a phase to a precision of about three degrees. A dual-portread/write memory 42 circuit is utilized to store the waveform data,which allows the waveform data to be written into the memory 42 andupdated if necessary by a processor (not shown) coupled to the memory42. Counter circuits 44 are provided so that a 10 MHZ local oscillator46 cycles through 122 memory locations at a rate, 40.98 kHz,sufficiently close to the resonant frequency, 41 kHz, of the selectedtransducer elements. The waveforms are encoded in these 122 memorylocations, and drive circuits 48 are used to convert the digitalwaveform data pulses into the high voltage needed to drive theparticular transducers.

FIG. 12 graphically illustrates the beam pointing ability for the 41 kHzresonator array illustrated in FIG. 10. In this example, the relativephases of the eight array sources were adjusted to give maximum signalstrength directly in front of the array, at 0 degrees; and then at adirection 18 degrees away from the plane of the resonator columns. Theability of the array to direct the acoustic signals in these directionsis demonstrated in this figure.

FIG. 13 is an example of the performance of a direction and rangingsystem built from a 41 kHz linear acoustic array illustrated in FIG. 10,the electronic driver circuitry illustrated in FIG. 11, and anultrasonic receiver. The receiver was in proximity to the acousticarray, and it received ultrasonic energy reflected from nearby roomobjects. The range to an object was determined by the transit time of a1 msec acoustic wave pulse from the linear array. The beam from thelinear array was steered into eight vertical directions separated byabout five degrees. The figure represents an intensity map for a chairat far range.

The invention provides acoustic data indicative of range, angulardirection and angular extent of an object within a scanned field. Basedon this data, the system processor 18 can determine range based on thetime required to receive the acoustic signal, angular direction based onthe angle of the steered beam when the acoustic signal is detected, andangular extent based on the arc of the steered beam during which theacoustic signal is detected. This information can be utilized in anoccupant detection system to control deployment of an air bag andprevent unwanted injuries. For example, the rate of inflation of the airbag may be made dependent on the range of the occupant. Further, theangular extent of the occupant can be utilized to control the force ofdeployment, so that occupants of smaller stature or children are notsubjected to the maximum deployment force of the air bag.

FIG. 14 illustrates an occupant detection system in accordance with theinvention that include a sensor module 50 mounted to a dashboard 52 of avehicle 54. The sensor module 50 includes the array of acoustic outputtransducers 12 and input transducer 20 discussed above, and ispositioned so that a steered acoustic beam emitted from the array ofacoustic output transducers 12 is directed to a passenger seat 56located within a passenger compartment 58 of the vehicle 54. The sensormodule 50 is coupled to a controller 60 that contains the variousprocessing circuitry previously described that is required to drive thearray of acoustic output transducers 12 and to analyze the signalsreceived from the input transducer 20. The controller 60 controls theactivation of the air bag 62 in response to the signals received fromthe input transducer 20 of the sensor module 50, such that the air bag62 is deployed within a prescribed envelope 64 within the compartment58.

The invention has been described with reference to certain preferredembodiments thereof. It will be understood, however, that modificationand variations are possible within the scope of the appended claims. Forexample, the sensor module may be located at positions within thepassenger compartment other than the dashboard, and may also be employedto detect objects in compartments other than the passenger compartment,as in the case of a child trapped in the trunk of a vehicle. Also, thenumber of transducers utilized in the array may vary along with theiroperating frequency. Still further, multiple modules may be employed sothat steered beams are generated in more than one plane.

What is claimed is:
 1. An acoustic wave transmitter comprising: aresonator body; a plurality of air chamber resonators arranged in anarray in the resonator body to form an array of exit holes; a pluralityof transducers corresponding to the plurality of air chamber resonators,wherein the transducers are acoustically coupled to the air chamberresonators and drive the air chamber resonator to generate an acousticsignal; wherein a spacing provided between adjacent exit holes is notgreater than one wavelength of the acoustic signal; and tuning means fortuning a resonance and phase of each of said air chamber resonators. 2.An acoustic wave transmitter as claimed in claim 1, wherein the airchamber resonators extend through the resonator body to form a secondarray of exit holes, wherein the tuning means comprises moveable plugslocated in the exit holes of the second array.
 3. An acoustic wavetransmitter as claimed in claim 1, wherein the transducers are resonantdiaphragm type transducers.
 4. An acoustic wave transmitter as claimedin claim 1, wherein the transducers are acoustically coupled to the airchamber resonators via transducer excitation holes formed in theresonator body.
 5. An acoustic wave transmitter as claimed in claim 4,wherein the transducer excitation holes are formed on opposite sides ofthe resonator body.
 6. An acoustic wave transmitter as claimed in claim4, wherein the transducer excitation holes are formed on a common sideof the resonator body.
 7. An acoustic wave transmitter comprising: aresonator body; a plurality of air chamber resonators arranged in anarray in the resonator body to form an array of exit holes; a pluralityof transducers corresponding to the plurality of air chamber resonators,wherein the transducers are acoustically coupled to the air chamberresonators and drive the air chamber resonator to generate an acousticsignal; a control circuit that converts waveform data stored in a memoryelement to drive electrical drive signals that are supplied to thetransducers; and wherein a spacing provided between adjacent exit holesis not greater than one wavelength of the acoustic signal.
 8. Anacoustic wave transmitter as claimed in claim 7, further comprising areceiver that receives the acoustic signal generated by the air chamberresonators.
 9. An acoustic wave transmitter as claimed in claim 8,further comprising a processor coupled to the receiver, wherein theprocessor determines at least one of range, angular extent and angulardirection of objects that reflect the acoustic signal.
 10. An acousticwave transmitter as claimed in claim 7, wherein the control circuitincludes: an addressable memory containing digital waveform data fordriving each air chamber resonator; a register for selecting regions ofthe addressable memory in which the waveform data reside; and a counterfor cycling through the selected regions to present the waveform data tothe transducers at a desired rate.
 11. An acoustic wave transmittercomprising: a resonator body; a plurality of air chamber resonatorsarranged in an array in the resonator body to form an array of exitholes; a plurality of transducers corresponding to the plurality of airchamber resonators, wherein the transducers are acoustically coupled tothe air chamber resonators and drive the air chamber resonator togenerate an acoustic signal; wherein a spacing provided between adjacentexit holes is not greater than one wavelength of the acoustic signal;and wherein the transducers drive the chamber resonators in relativephase to each other to produce a steered acoustic beam as the acousticsignal.
 12. A detection apparatus for detecting an object within acompartment of a vehicle: an acoustic wave transmitter that generates asteered acoustic signal; a receiver that receives a reflected acousticsignal generated by the reflection of the steered acoustic signal offobjects located within the vehicle compartment; a controller coupled tothe receiver; wherein the controller, using the reflected steeredacoustic signal, determines at least one of range, angular extent andangular direction of the object; wherein the acoustic wave transmitterincludes: a resonator body; a plurality of air chamber resonatorsarranged in an array in the resonator body to form an array of exitholes; and a plurality of transducers corresponding to the plurality ofair chamber resonators, wherein the transducers are acoustically coupledto the air chamber resonators and drive the air chamber resonators togenerate the steered acoustic signal.
 13. A detection apparatus asclaimed in claim 12, wherein a spacing provided between adjacent exitholes is not greater than one wavelength of the acoustic signal.
 14. Adetection apparatus as claimed in claim 12, further comprising: tuningmeans for tuning a resonance and phase of each of said air chamberresonators.
 15. A detection apparatus as claimed in claim 14, whereinthe air chamber resonators extend through the resonator body to form asecond array of exit holes, and wherein the tuning means comprisesmoveable plugs located in the exit holes of the second array.
 16. Adetection apparatus as claimed in claim 12, wherein the transducers areresonant diaphragm type transducers.
 17. A detection apparatus asclaimed in claim 12, wherein the controller includes: an addressablememory containing digital waveform data for driving each air chamberresonator; a register for selecting regions of the addressable memory inwhich the waveform data reside; and a counter for cycling through theselected regions to present the waveform data to the transducers at adesired rate.